Flat VMDK on vSAN

Products

• VMware vSAN

Issue/Introduction

Symptoms:

A Virtual Machine is running from a flat VMDK on vSAN.

Any reconfiguration to the VM's disk fails with error "Error: A general system error occurred: PBM error occurred during PreReconfigureCallback: Fault cause: vmodl.fault.SystemError"

Environment

VMware vSAN 8.0.x
VMware vSAN 7.0.x

Cause

During the normal storage vMotion from traditional to vSAN datastore, the flat file gets converted into a vSAN object. The flat file is not converted if the VM files are manual copied from traditional storage to vSAN.

Traditional Storage:-
Descriptor file.vmdk
flat.vmdk

vSAN:-
Descriptor file.vmdk
vSAN object UUID

In VSAN's object-based storage, -flat.vmdk and -delta.vmdk files are not directly addressable as datastore paths.

Resolution

Migrate the VM to traditional storage, then migrate it back to vSAN using storage migration (offline or SvMotion) from vCenter to ensure the conversion to vSAN objects occurs.

The below video can assist with migrating VMs from traditional storage to vSAN.

Additional Information

• Failed to power on virtual machine
• Having "flat" vmdk's in VSAN datastore can cause various types of consumption issues that can result in the disks on hosts to fill up unexpectedly and cause hosts to become non-responsive.
• Having VMs running on flat.vmdk in vSAN can also prevent you from expanding the VM disks and cause them to read as 0 in size when checked via Edit settings on the VM.
• This also could prevent VMs from powering on if they consume all the space available in the namespace.
• When trying to modify/add virtual machine configurations like HDD etc. the task fails.